Polymer-electrolyte membrane fuel cell

ABSTRACT

Not all gas diffusion structures known to date can guarantee water balance compensation in the fuel cell, protection against drying and prevention of water deposits while simultaneously ensuring even distribution of reaction gases. In the gas diffusion structure, according to the invention, a gradient is produced in terms of gas permeability perpendicular to the membrane by way of a stratified system. This guarantees, at an appropriate operating temperature and appropriate stoichiometry of the reactants, constant and optimal humidification in all points of the membrane by the formed product water. Additional humidification systems can thus be discarded.

BACKGROUND OF THE INVENTION

The present invention relates to a polymer-electrolyte membrane fuelcell comprising a laminate of such membrane, an electrode containing acatalyst, a porous, electrically conductive gas diffusion layer and acurrent collector plate having a gas distribution channel structure, thecell having a gradient of the gas permeability, which gradient ispresent at least in a partial area, in the laminate in the directionperpendicularly to the membrane, with a higher gas permeability closerto the membrane an a lower gas permeability closer to the currentcollector plate, wherein in operation at the membrane by ahydrogen-oxygen reaction water and heat are produced. The inventionfurther relates to a method for manufacturing such fuel cell.

The polymer-electrolyte membrane fuel cell or PEM fuel cell is operatedwith the reaction gases hydrogen and oxygen or air. According to a knownstructure (EP 0560295A), it consists of, arranged from the outside tothe inside, two current collector plates with gas diffusion layers, twoelectrodes containing the catalyst and an ion exchange membrane locatedbetween the electrodes, with the ion exchange membrane and theelectrodes forming the membrane-electrode-unit (hereinafter referred toas MEA—membrane electrode assembly). Typically the current collectorplates contain structures than extend parallel to the MEA for supplyingand distributing the reactants across the entire surface of the cell.Since the voltage of each individual cell is much too low for anypractical applications, a multitude of these cells must be seriallyarranged. In the resulting fuel cell pile or fuel cell stack, thecoinciding current collector plates are replaced with so-called bipolarplates whose function consists of directing the current though the stackand of isolating the reaction gases.

By supplying hydrogen which is a typical reaction gas to the fuel cellanode side which is positioned in a gas compartment sealed off to theoutside, cations are generated on the anode side catalyst layer whichcations diffuse though the ion exchange membrane. Simultaneously, theelectrons that were also produced are directed via an outer electriccircuit with a load resistor from the anode to the cathode. The suppliedoxidation agent is oxygen, and preferably the oxygen concentration inambient air is sufficient; this oxygen is now reduced in the cathode byway of reception of the hydrogen ions and electrons. Water results as areaction product. The reaction enthalpy is released in the form ofelectric energy and dissipated heat.

An essential problem in this process is the water economy of the fuelcell. In the initially mentioned known fuel cell (EP 0560295A) the watereconomy particularly in the electrode layer which contains the catalystis influenced by means of hydrophobic coatings covering the catalystcarrier which itself is covered by the catalyst. In order that thecatalyst still comes into contact with the reactants, in this layer theporosity increases towards the membrane. In the area of low porosity ahigher percentage, in the area of the high porosity a lower percentageof the catalyst is inactive. The water generation takes place over allof the thickness of the layer and can also deactivate parts of thecatalyst by flooding. On the other hand, the membrane is able to performunder optimal conditions, i.e. it conducts the hydrogen ions optionally,only if it contains a sufficient amount of moisture. If the moisturecontent drops too low, the internal resistance of the cell increasesconsiderably due to the increased membrane resistance, thereby reducingperformance. Thus, for an optimal cell operation at a given temperatureit is necessary that the air's humidity is at almost 100 percent at eachand every place of the membrane. If the cathode gas air flows throughthe distribution channels of the current collector plates and diffusesthrough the gas diffusion layer, it has a low partial pressure withregard to water vapor upon entering the gas compartment, and a high oneupon exiting because oxygen reacts to form water at the cathode. Thediffusion flow between the membrane's surface and the distributionchannel, which is caused by the partial pressure differences ofhydrogen, dries the membrane at the entry point of the cathode gas, andat the exit point, on the other hand, water deposits may occur in thediffusion layer. At a given operating temperature and in order tocompensate for differences in the water balance of the membrane it istherefore necessary to achieve a composition of the cathode gas that isas constant as possible across the membrane's surface; the same appliesfor the anode gas.

With conventional methods this problem is only partially solved usingexternal humidification systems, at times in combination with coolingsystems, which systems, by way of measuring the membrane moisture atleast in intervals, provide for water balance adjustments of the cell.It is a disadvantageous aspect of these humidification systems that theyplace an additional burden on the fuel cell system in terms of internalenergy consumption and also weight, which is particularly undesirablefor their application in small, portable systems, as well as in terms ofadditional cost thereby reducing the competitiveness of the fuel cell incomparison with conventional energy supply systems. Furthermore, theabove solution does not address the problem of how to achieve an evengas distribution on the surface of the catalyst and of the cellmembrane. The objective is to operate a fuel cell without humidifyingthe reaction gases.

Electrode-catalyst layers which are porous and such allow some gasdiffusion are also known from WO 97/20359. These layers may also consistof a laminate of several films and can be reinforced by a conductivegrid. However, they do not have a gradient of the gas permeability.

U.S. Pat. No. 5,641,586 shows one solution as to how to achieve auniform distribution of the reaction gases. It provides that two layersare arranged between the electrode catalyst layer and the currentcollector plate; adjacent to the MEA is a macro-porous, hydrophile gasdiffusion layer, and adjacent to the current collector plate is amacro-porous, hydrophilic flow field. The flow field has twointermeshing channel structures, on the one hand, for distributing thereaction gases at the gas diffusion layers and, on the other hand, forremoving the reaction products. Although this apparatus allows the evendistribution of the reactants across the surface of the membrane andwater deposits are also avoided by way of the application of ahydrophobic layer, the danger of drying continues to remain a threatbecause the gas diffusion layer does not prevent the reaction productfrom exiting even if the membrane has an insufficient moisture supply.

One way for maintaining the water economy of a fuel cell on a constantand optimal level is shown in DE-OS 14 96 172, according to which awater diffusion electrode made of a palladium/gold alloy is used whichis permeable for hydrogen but prevents the penetration of fluids.However, this design is not suitable for use at the cathode, where theproblem of having to compensate for water imbalances primarily is sincethis electrode is not permeable for oxygen.

SUMMARY OF THE INVENTION

Thus, the object of the present invention is to replace the conventionalgas diffusion layer of the PEM fuel cells with a gas diffusion structurewhose diffusion properties are dimensioned in such a way that the waterbalance at the MEA is always uniform.

The fuel cell of the present invention is to be manufactured by asimple, inexpensive method for producing such gas diffusion structure.

According to the invention these objects are accomplished by a gasdiffusion structure which is characterized in that the gas permeabilitygradient exists in the gas diffusion layer which is adjacent to theelectrode containing the catalyst, and that at least in said partialarea of the gas diffusion layer the gas permeability closer to thecurrent collector plate is lower to such a degree than in the vicinityof the membrane that a gas composition occurs which over the surface ofthe membrane is approximately constant, and the water which at theoperating temperature is generated in vaporform creates such a watervapor diffusion stream through the gas diffusion layer that such ahumidity content of the membrane which is optimal for the conductivityis materially being maintained, and by a method for producing the gasdiffusion structure, which method is characterized in that the gasdiffusion layer is produced with a gas permeability increasing in thedirection toward the membrane. The gradient is thus provided for in thegas diffusion layer which for the purpose of gas diffusion must have acertain thickness and wherein the generation of the water takes placebeyond the delimiting face oriented toward the catalyst. The water vapordefuses in an administered stream of vapor out of the inner zone of thecell in such a manner that along all of the membrane approximately equalgas access conditions and humidity conditions exist which conditions canbe adjusted to be optimum.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing shows a preferred embodiment of a gas diffusion structureaccording to the invention in correspondence with the examples.

DETAILED DESCRIPTION

The depicted fuel cell comprises a current collector plate 1 havingchannels 2 for the distribution of the reaction gases, an outer layer 3and an inner layer 4 of a gas diffusion structure, an electrode 5 havinga catalyst film, and a polymer-electro-lyte membrane 6; at the oppositeside of this membrane 6, non-depicted cathodic structural elementsfollow.

Normally the gas diffusion structure 3, 4 consists of porous,electrically conductive materials and is an orthogonale structure thatmust be adapted with respect to their pore volumes in order to achieve agradient in terms of gas permeability. This can be accomplished inlayers or continually. An almost continual change could be accomplishedif a large number of very thin layers with respectively somewhat varyingdiffusion properties are laminated; at a minimum, and subsequently alsopreferably, two layers 3, 4 with different gas permeability propertiesare required.

The outer layer 3, i.e. the part of the gas diffusion structure which islocated adjacent to the current collector plate 1 having the channels 2,has a relatively small pore volume and consequently a high diffusionresistance. The inner part 4 of the gas diffusion structure, however,i.e. the part that contacts the electrode 5 which contains the catalyst,has a relatively high pore volume. Therefore, the reaction gases getdistributed sufficiently well by way of diffusion within the layer 4,also at distances (0.5 to 5 mm) approximately in range of the projectingribs of the current collector plate 1. Moreover, the outer layer 3 ofthe orthogonal gas diffusion structure is preferably realized as morehydrophobic than the inner layer 4.

This two-layered gas diffusion structure can further be supplementedwith additional layers that have respectively different functions.Adding another layer to the inner layer 4 between the electrode 5 andthis layer 4, can be indeed advantageous. The purpose for suchadditional layer is the superficial filling and smoothing of the formostly coarse pores of the layer 4 which provides a better contactbetween the gas diffusion structure and the electrode. A method formanufacturing this layer is described in example 3.

Also, another layer can be added between the outside layer 3 and theprojecting ribs of the current collector plate 1. The purpose for such alayer consists of keeping the electric contact resistance to the channelstructure as low as possible. For this, the layer could be deformable byplasticity or elasticity thereby allowing that the dimensionaltolerances of the current collector plates or, in the case of anarrangement in a fuel cell stack, of the bipolar plates are compensatedfor and the current collection from the gas diffusion structure canoccur evenly.

Subsequently, the way in which the gas diffusion structure consisting ofboth layers 3 and 4 functions, will be outlined. The adjustableoperational parameters of the fuel cell are the air ratio [ratio airstream: air chemically consumed] or the hydrogen stoichiometry, theoperating pressure, the water vapor content of the reaction gases at thecell entry point and the operating temperature. The temperature shouldbe chosen as high as possible in order to allow for a compact coolingsystem with low coolant throughput. The simplest, conceivable coolingsystem consists merely of a fan that transports a amount of airsufficient for the cooling purpose into the cathode compartment of thecell or of the stack. The adjustable operating pressure should be chosena low a possible; ideally the fuel cell should be operated at ambientpressure. Thus, compressor-related energy losses can be avoided.Moreover, this represents a reduction in weight and expense.

During operation in accordance with these or similar conditions theproblem focus with regard to a uniform water balance is placed primarilyon the danger of the membrane to dry out and not on water deposits inthe pores of the gas diffusion structure. For purposes of preventingsuch dry out, in particular on the cathode side an approximatelyconstant gas composition across the entire surface of the membrane andan approximately constant gas composition in the channel structure ofthe current collector plate is demanded. This is accomplished byadopting a relatively large air ratio (e.g. 8 to 70) through thechannels 2 of the current collector plate 1. Under these conditions,changes of the composition of the gas due to the withdrawal of oxygenand the release of water vapor by the cell are negligible. The diffusionresistance of the layer 3 of the gas diffusion structure according tothe invention must be such that the gradient in the water vapor'spartial pressure, occurring at the desired operating temperature betweenthe well-moistened membrane and the gas in the cathode compartmentcauses a water vapor diffusion flow that carries away just the generatedproduct water. Therefore, the essential oxygen and water vapor gradientsdo not occur, as is customary, in a parallel direction but in a verticaldirection in relation to the surface of the membrane 6. The operatingconditions are thus constant across the entire surface of the membrane.The layer 4 has the purpose to distribute the gases uniformly at thatarea of the projecting ribs wherein the diffusion flow is interrupted.

If a relatively low air ratio is desired which causes changes in thecomposition of the cathode gas on its way through the cell to benoticeable, the diffusion resistance of the outer layer 3 must beadjusted to these conditions. At the point of gas entry the layer 3 mustbe realized with a higher diffusion resistance than at the gas exitpoint. In addition, in this manner a performance reduction based onchanges of the operating temperature inside of cell can be avoided by anadjustment of the diffusion resistance.

The gas diffusion structure of the invention is suited for advantageousapplication not only at the cathode, as has been described here by wayof example, but also at the anode, in particular if the cell is operatedwith non-moistened hydrogen.

The production of such orthogonal gas diffusion structure on thecatalytic electrode, with the gradient in terms of gas permeability, isdescribed in the following upon use of examples.

EXAMPLE 1

The gradient in terms of gas permeability perpendicular to the membrane6, is accomplished by hot-pressing a foil consisting of a thermoplasticsynthetic material supposed to constitute the layer 3 onto a carbonfiber paper 4 which is homogenous in terms of its diffusion propertiesand which is supposed to constitute the layer 4. The hot-pressing causesthe thermoplastic synthetic material to be distributed inside the poresclose to the one surface of the carbon fiber paper and results in apartial obstruction of the pores. Surprisingly, the electricconductivity perpendicular to the layer is not lost thereby because thecarbon fibers penetrate the foil and the electric current thereforereaches the surface again.

Preferably, the initial pore volume of the carbon fiber paper is morethan 50%, and in particular, preferably it exceeds 70%. Before it isprocessed further, it can be impregnated with polytetrafluoroethylene(PTFE) (5 to 40 mass %) in accordance with the method described inexample 2. The thickness of the carbon fiber paper is preferably 0.1 mmto 1.5 mm, and in particular preferably 0.2 mm to 0.4 mm.

In order to render the foil of the thermoplastic material hydrophobicthey consist preferably of fluorinated synthetic materials, such as theproducts e.g. THV and FEP of the Dyneon company. But also suitable foruse are conventional thermoplastics, such as polypropylene. Thethickness of the foil is preferably 0.01 mm to 0.2 mm. The diffusionresistance can be adjusted within wide limits, depending on the intendedpurpose, by choosing the foil thickness and the pressing conditions.

The temperature during hot-pressing is preferably selected in a rangethat is somewhat above to somewhat below the melting range of the usedthermoplastic. The pressing pressures are preferably between 10 bar and100 bar, in particular preferred are pressures of 30 bar to 80 bar. Ifpossible, the fibers of the carbon fiber paper should not break due tothe pressing.

The other surface of the carbon fiber paper can be coated with anelectrode of a catalyst-containing material, or it can be placed orpressed directly onto a previously catalyzed membrane. In the lattercase, however, it is advantageous to apply a smoothing layercompensating for the surface roughness of the carbon fiber paper, asoutlined in example 3.

EXAMPLE 2

The pores, in part obstructed with a synthetic material, in accordancewith example 1, can also be produced by drenching a carbon fiber paperwith suspended synthetic materials, preferably PTFE or THV. Since it isvery complicated to achieve a gradient in perpendicular direction to thepaper's surface by way of drenching in only one single carbon fiberpaper, two carbon fiber papers are to be used, one of which which issupposed to constitute the outer layer 3 containing a relatively highamount of synthetic material and thus resulting in a higher diffusionresistance, and the other one of which which is supposed to constitutethe inner layer 4 containing a relatively high number of unobstructedpores. The second carbon fiber paper can then be equipped with acatalyst-containing electrode layer or with a layer compensating for theroughness of the carbon fiber paper, as shown in example 3, and can thenbe placed or pressed correspondingly onto the membrane which may benon-catalyzed or catalyzed. The specifications referred to in example 1apply for both carbon fiber papers here as well.

Specifically for the present example two carbon fiber papers with athickness of 0.17 mm for the first layer 3 and of 0.35 mm for the secondlayer 4, manufactured by the Toray company (Japan), are used. The firstpaper is partially filled with synthetic material by drenching with a60% aqueous PTFE dispersion, followed by drying at a higher temperature.A single process step consisting of drenching and drying, however, isgenerally not sufficient to achieve the desired diffusion resistance,and repeating this process step only adds minimal amounts only to thecarbon fiber paper since the hydrophobic synthetic material alreadyadsorbed prevents for the most part any further penetration of thedispersion into the pores. Higher filling rates can only be achieved byvacuum and pressure treatments, performed subsequently one after theother, during the impregnating process. After the carbon fiber paper isimpregnated the remaining surface-active agents from the dispersion arethermically destroyed, for which purpose temperatures of between 300° C.and 400° C. are typically applied for a short time.

The completed layer is extremely hydrophobic. It contains 45% to 75%synthetic material if applied for a gas diffusion structure on thecathode side of a PEM fuel cell to be operated at ambient pressure andat a 70° C. cell temperature with air cooling of the cathodecompartment. The conductivity of the layer is not essentially affectedby this because inside the carbon fiber paper the conductive connectionsbetween the carbon fibers are not dissolved. The second carbon fiberpaper, which may be treated to have slight hydrophobic properties, ispress-compacted with the first paper or it is only added during theassembly of the cell.

EXAMPLE 3

A laminate system with very good electric conductivity properties, whichhas a suitable gradient in the diffusion resistance, can also bemanufactured by applying a mixture consisting of an electricallyconductible powder and of a binder to a substrate with a low diffusionresistance, e.g. carbon fiber paper.

Specifically for this example a carbon fiber paper, treated to exhibithydrophobic properties, with a pore volume of 68% and a thickness of0.35 mm is used. In general, carbon fiber papers with the specificationsreferred to in example 1 are adequate. A dispersion of graphite powder,THV (Dyneon company) or PTFE (e.g. Hostaflon TF 5032) in suitableaqueous dispersion fluids is sprayed onto this substrate in one orseveral spraying steps while allowing time to dry between the sprayingsteps. Aqueous tenside solutions or mixtures consisting of water andtypes of alcohol can be used as dispersion fluids. Carbon powder ispreferably suitable as an electrically conductible powder, and inparticular preferred are globular-shaped carbon particles, such as e.g.Mesocarbon Microbeads obtainable from the company Osaka Gas, Japan.

Preferably the spraying is carried out on a vacuum table allowing for animmediate removal of excess dispersion fluid by suction. After the lastdrying step the synthetic particles are sintered at increasedtemperatures, and the surface-active agents are destroyed. Thepercentage of synthetic binder is preferably 5% to 50% of the dry mass.The area specific mass of the layer is preferably between 30 g/m² and300 g/m², particularly preferred between 60 g/m² and 120 g/m². After thesintering process is complete the substrate with the applied layer arepress-compacted at between 5 bar and 100 bar, preferably at 30 bar to 80bar, at an increased temperature.

To compensate for the roughness of the carbon fiber paper on the sidefacing the membrane or the catalyst, it is useful to apply a very thinlayer of a dispersion consisting of porous carbon black (e.g. Vulcan XC72 by the Cabot company) and a polymeric binder (e.g. PTFE) by way ofspraying, possibly on the vacuum table, drying and subsequent sintering.Again, the preferred rate of synthetic material is 5% to 50%. This layercan also be press-compacted using the pressures indicated above.

EXAMPLE 4

For purposes of impregnating the carbon fiber paper, as a variation ofexample 2, it is possible to use, instead of the fluorinated syntheticmaterial, a mixture of electrically conductive particles with an e.g.fluorinated synthetic material as a binder. The advantage is thesomewhat superior electric conductivity of the gas diffusion structure.

Suitable electrically conductive particles are graphite, conductivecarbon black or short carbon fibers. Specifically for the presentexample a suspension consisting of 50 g water, 16.6 g of 60% PTFEsuspension and 10 g graphite with a medium particle size of 15 μm can beused for impregnating. After the appropriate solid matter mass for theapplication purpose (e.g. 2 mg/cm² to 10 mg/cm²) is achieved the layermaterial is sintered in order to solidify the structure and to thermallydisintegrate the supplementary dispersion materials. The impregnationprocess, accompanied by intermittent drying, can be repeated severaltimes.

If also in parallel to the layer surface, a gradient in the diffusionresistance is desired due to the specific application, such gradient caneasily be achieved by way of applying respectively different numbers ofimpregnation steps on different partial areas. Preferred in this case isthe impregnation of partial areas of the diffusion electrode by way ofspray application of the suspension and allowing it to soak inaccordance with example 2 or example 4.

The gradient in the diffusion resistance perpendicularly to the layercan be achieved by attaching another carbon fiber paper either on theside facing the membrane or on the side off from it.

In the alternative, a layer applied in accordance with example 3 andconsisting of highly porous carbon black can also be used for thispurpose. Possible application techniques are spraying, applying bydoctor, rolling or screen print. Preferably herein, solid material loadsof 0.4 mg/cm² to 3.5 mg/cm² are used.

REFERENCE LIST

(1) current collector plate

(2) channels for distributing the reaction gases

(3) outer layers of the orthogonal gas diffusion structure

(4) inner layers of the orthogonal gas diffusion structure

(5) catalyst-containing electrode

(6) polymerelectrolyte membrane

What is claimed is:
 1. A polymer-electrolyte membrane fuel cellcomprising a laminate of such membrane (6), an electrode (5) containinga catalyst, a porous, electrically conductive gas diffusion layer (3, 4)and a current collector plate (1) having a gas distribution channelstructure (2), the cell having a gradient of the gas permeability, whichgradient is present at least in a partial area, in the laminate in thedirection perpendicularly to the membrane, with a higher gaspermeability closer to the membrane (6) an a lower gas permeabilitycloser to the current collector plate (1), wherein in operation at themembrane by a hydrogen-oxygen reaction water and heat are produced,characterized in that the gas permeability gradient exists in the gasdiffusion layer (3, 4) which is adjacent to the electrode (5) containingthe catalyst, and that at least in said partial area of the gasdiffusion layer (3, 4) the gas permeability closer to the currentcollector plate (1) is lower to such a degree than in the vicinity ofthe membrane (6) that a gas composition occurs which over the surface ofthe membrane (6) is approximately constant, and the water which at theoperating temperature is generated in vaporform creates such a watervapor diffusion stream through the gas diffusion layer that such ahumidity content of the membrane which is optimal for the conductivityis materially being maintained.
 2. Fuel cell according to claim 1,characterized in that the electrically conductive gas diffusion layer(3, 4) consists of a plurality of partial layers (3; 4) following eachother along the thickness, the specific gas permeabilities of thesepartial layers increasing towards the side of the membrane from onepartial layer to the next.
 3. Fuel cell according to claim 1,characterized in that the electrically conductive gas diffusion layer(3, 4) has a higher pore volume dentity in the zones with the higher gaspermeability than in the zones with the low gas permeability.
 4. Fuelcell according to claim 1, characterized in that the gas diffusion layer(3, 4) has a gas permeability gradient also in a direction parallel tothe membrane (6).
 5. Method for producing a polymer electrolyte membranefuel cell according to claim 1, the cell comprising a porous,electrically conductive gas diffusion layer (3, 4) which is arrangedalong such a membrane with an electrode (5) containing a catalyst beingarranged inbetween, the method comprising producing that gas diffusionlayer (3, 4) with a gas permeability gradient in the direction of itsthickness, characterized in that the gas diffusion layer (3, 4) isproduced with a gas permeability increasing in the direction toward themembrane (6).
 6. Method according to claim 5, characterized in that thegas diffusion layer is produced in the form of at least two partiallayers (3, 4) with different gas permeabilities, and that in the fuelcell the partial layer or layers having the higher gas permeability arelocated at the membrane.
 7. Method according to claim 6, characterizedin that the different gas permeabilities are obtained by producingdifferent pore volume densities.
 8. Method according to claim 5,characterized in that the gas diffusion layer is produced of porouscarbon fibre paper and that for obtaining the different gaspermeability, the pores of the carbon fibre paper near one of itssurfaces is partially closed by a thermoplastic synthetic material byputting onto this surface a foil while exerting pressure and elevatedtemperature.
 9. Method according to claim 5, characterized in that forobtaining the different gas permeabilities, starting out from a layermaterial having uniform gas permeability, the pores of the porous gasdiffusion layer are partially closed with a suspension of plasticparticles.
 10. Method according to claim 9, characterized in that thegas diffusion layer is produced of porous carbon fibre paper and thesuspension is introduced into the carbon fibre paper by drenching anddrying.
 11. Method according to claim 5, characterized in that forobtaining the non-uniform gas permeabilities, starting out from a layermaterial having uniform gas permeability, at one of the surfaces of thegas diffusion layer a further layer of electrically conductive particlesand binder particles are deposited.
 12. Method according to claim 11,characterized in that the further layer is deposited by suspending theconductive particles which preferably consist of carbon and the binderparticles which preferably consist of fluorinated synthetic material ina suspension and spraying this suspension on the respective surface ofthe gas diffusion layer, then drying and possibly sintering the layer.